Prolonged pain appears to have the potential to modulate its own intensity through positive and negative feedback (central sensitization by pain, pain inhibition by pain). If the feedback is positive, the result is a vicious pain cycle and a progressive increase in pain sensitivity. Increased pain sensitivity means that a given stimulus is perceived as more painful (hyperalgesia), or - in more extreme cases - that a previously non-painful stimulus becomes painful (allodynia). The vicious cycle may lead to sensitization beyond the topographical boundaries of the original pain, and thus it may render remote body regions more pain-prone. The result may be a snowball effect of progressive expansion of the painful area. There is evidence suggesting that the vicious cycle may be a pathophysiological factor in certain chronic pain diseases, including fibromyalgia syndrome (FMS), myofascial pain syndrome (MPS), and irritable bowel syndrome (IBS). This research is guided by 4 questions: 1) Does the intensity and duration of a persistent pain have an effect on how pain sensitivity changes over time? 2) Does the sensitizing effect of pain signals reach beyond the topographical location of the original pain focus? 3) Is it possible to interrupt the vicious pain cycle and allow the sensitized state to return to normal by temporarily silencing the local pain focus that presumably started the cycle? 4) Does the maintenance of the sensitized state depend on central NMDA receptor function, molecular constituents known to play a role in temporal integration of pain stimuli and other memory systems? The subjects in this study will be asked to rate pain intensities by setting the slider on an electronic visual analog scale. Novel methodology will be used for probing the temporal and spatial response properties of central pain modulation with experimental pain with prolonged series of thermal pulses. The effect of silencing clinical pain foci with transdermally delivered local anesthetics on thermallyinduced sensitization will be studied. The importance of NMDA receptor systems in the maintenance of a sensitized state will be assessed by measuring pain sensitization properties before and after pharmacologically blocking them.